Exhaust parts such as exhaust manifolds, exhaust pipes, converter cases, and mufflers of automobiles are required to have excellent oxidation resistance, an excellent thermal fatigue resistance, and an excellent high-temperature fatigue resistance (hereinafter these properties are generally referred to as “heat resistance”). Specifically, the meaning of thermal fatigue and high-temperature fatigue is as follows. In the description below concerning the composition, “%” means “% by mass”.
Exhaust parts are under restraint with respect to surrounding parts when they are repeatedly heated and cooled as the engine is started and stopped. Thus, thermal expansion and contraction of the exhaust parts are limited and thermal strain is generated in the material of these parts. The fatigue phenomenon attributable to this thermal strain is called thermal fatigue.
High-temperature fatigue is a phenomenon in which parts subjected to continuous vibration while being heated by exhaust gas from engines reach fracture, such as cracking.
Currently, as the material for parts required to have such a heat resistance, Cr-containing steels such as Type 429 steels containing Nb and Si (15% Cr-0.9% Si-0.4% Nb: for example, JFE 429EX) are widely used. However, with improvements in engine performance, exhaust gas reaches a temperature higher than 900° C. At such a temperature, the Type 429 steels may satisfy the required properties but may not sufficiently satisfy a required thermal fatigue resistance in particular.
Examples of the raw material developed to address this issue include SUS 444 steels (for example, 19% Cr—Nb-2% Mo) prescribed in JIS G4305 in which Mo as well as Nb is added to improve high-temperature resistance and ferritic stainless steels containing Nb, Mo, and W (for example, refer to Patent Literature 1). Due to recent extraordinary escalation and fluctuation in price of rare metals such as Mo and W, development of materials that have a comparable heat resistance but use inexpensive raw materials has become desirable.
An example of a material that has an excellent heat resistance but does not contain expensive Mo or W is a ferritic stainless steel for use in automobile exhaust gas flow channels disclosed in Patent Literature 2. This ferritic stainless steel is obtained by adding Nb: 0.50% or less, Cu: 0.8% to 2.0%, and V: 0.03% to 0.20% to a Cr-containing steel having a Cr content of 10% to 20%. Patent Literature 3 discloses a ferritic stainless steel that has an excellent thermal fatigue resistance obtained by adding Ti: 0.05% to 0.30%, Nb: 0.10% to 0.60%, Cu: 0.8% to 2.0%, and B: 0.0005% to 0.02% to a Cr-containing steel having a Cr content of 10% to 20%. Patent Literature 4 discloses a ferritic stainless steel for use in automobile exhaust parts, obtained by adding Cu: 1% to 3% to a Cr-containing steel having a Cr content of 15% to 25%. The feature of these steels is that the thermal fatigue resistance is improved by adding Cu.
Another proposed approach for improving the heat resistance property is to intentionally add Al. For example, Patent Literature 5 discloses a ferritic stainless steel whose thermal fatigue resistance is enhanced by addition of Al: 0.2% to 2.5%, Nb: more than 0.5% to 1.0%, and Ti: 3×(C+N) % to 0.25%. Patent Literature 6 discloses a ferritic stainless steel whose oxidation resistance is improved by forming an Al2O3 film on a steel surface by addition of Al to a Cr-containing steel that contains Cr: 10% to 25%, and Ti: 3×(C+N) to 20×(C+N). Patent Literature 7 discloses a ferritic stainless steel whose post-hydroforming cracking resistance is improved by fixing C and N by addition of Ti, Nb, V, and Al to a Cr-containing steel having a Cr content of 6% to 25%. Patent Literature 8 discloses a steel having an excellent thermal fatigue resistance, excellent oxidation resistance, and an excellent high-temperature fatigue resistance obtained by adding Nb: 0.3% to 0.65%, Cu: 1.0% to 2.5%, and Al: 0.2% to 1.0% to a Cr-containing steel having a Cr content of 16% to 23%.